When standardized cat extracts were licensed, they were arbitrarily assigned 100,000 Allergy Units/ml if they contained 10-19.9 Fel d I u/ml. With the introduction of the ID50EAL method, biological Allergy Units are currently assigned based on skin test potency determined in highly skin reactive cat allergic subjects. Two standardized cat extracts widely differing in composition of non-Fel d I allergens, but containing equal Fel d I contents of 10 and 20 Fel d I u/ml were assayed to estimate the number of Allergy Units to be assigned to each. Both the 10 and 20 Fel d I containing extracts had mean D50s' between 11 and 12.9 and therefore merit assignment of 10,000 AU/ml rather than the 100,000 AU/ml arbitrarily assigned. The lower potency of standardized cat extracts might explain the higher number of Allergy Units associated with efficacy compared to pollen extracts containing 100,000 AU/ml and to the lower prevalence of skin test reactivity to standardized cat extracts labeled as 100,000 AU/ml compared to grass pollen extracts labeled 100,000 AU/ml.